The most common technique for error detection of non-real time services is based on Automatic Repeat request (ARQ) schemes which is combined with Forward Error Correction (FEC), called hybrid ARQ. If an error is detected by Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional bits. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit (PDU) will be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ are e.g. defined in S. Kallel, R. Link, S. Bakhtiyari, IEEE Transactions on Vehicular Technology, Vol. 48 #3, May 1999 “Throughput Performance of Memory ARQ Schemes”.                Type I: The erroneous PDU is discarded and a new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU.        Type II: The erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU's sometimes have higher coding rates and, are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.        Type III: Is the same as Type II only that every retransmitted PDU is now self-decodable. This implies that the PDU is decodable without the combination with previous PDU's. This is useful if some PDU's are so heavily damaged that almost no information is reusable.        
This invention is related to Type II and Type III schemes, where the received (re)transmissions are combined. These schemes can be seen as a link adaptation technique, since the redundancy can be adapted according to the channel conditions as for example described in 3GPP TSG RAN, “Physical Layer Aspects of High. Speed Downlink Packet Access TR25.848 V5.0.0” and in Amitava Ghosh, Louay Jalloul, Mark Cudak, Brian Casson, “Performance of Coded Higher Order Modulation and Hybrid ARQ for Next Generation Cellular CDMA Systems”, Proceedings of VTC 2000.
Another technique that falls under this category of link adaptation, is adaptive modulation and coding (AMC). A description of AMC can be found in the above-mentioned documents. The principle of AMC is to change the modulation and coding format in accordance with variations in the channel conditions or system restrictions. The channel conditions can be estimated e.g. based on feedback from the receiver. In a system with AMC, users in favorable positions e.g. users close to the cell site are typically assigned higher order modulation with higher code rates (e.g. 64 QAM with R=3/4 Turbo Codes), while users in unfavorable positions e.g. users close to the cell boundary, are assigned lower order modulation with lower code rates (e.g. QPSK with R=1/2 Turbo Codes).
In the following, different combinations of coding and modulation will be referred to as Modulation Coding Scheme (MCS) levels.
A transmission will be split into Transmission Time Intervals (TTI), whereas the MCS level could change each TTI interval (for HSDPA the TTI is equal to 2 ms).
Thus, depending on the channel conditions, different MCS levels can be scheduled. Packet size depends on MCS level and number of orthogonal codes allocated for a particular transmission. We will refer to MCS level and number of codes as Transport Format and Resource Combination (TFRC).
Apart from MCS used, bit combining method also influences the robustness of packets to the transmission errors.
There are different combining schemes, Chase Combining (CC) and Incremental Redundancy IR), that can be used for bit combining. In Chase Combining, always the same information and parity bits are sent to be combined and every version of packet is self decodable. The set of parity bits is always obtained by using the same puncturing scheme. Incremental Redundancy may use different sets of parity bits (obtained by different puncturing schemes) in consecutive packet transmissions. All these groups of obtained from different transmissions have to be stored in the soft buffer for combining. Hence, Incremental Redundancy provides more reliable transmission at the expense of increased soft buffer memory requirements.